Electron discharge tube testing circuit



1960 E. o. STONE 2,947,936

ELECTRON DISCHARGE TUBE TESTING CIRCUIT Filed May 15, 1957 u 4 a 19 :7 l I i FA "1 E I [Q4 58 x E Fl6.l.

2 9 E 3 P 5 \d INVENTOR ELMER O. STONE BY fi ti ATTO R N EY 2,947,936 ELECTRON DISCHARGE TUBE TESTING CIRCUIT Elmer 0. Stone, Seneca Falls, N.Y., assignor, by mesne assignments, to Sylvania Electric Products Inc., Wil- 'mmgton, DeL, a corporation'of-Delaware Filed May 15,1957, Ser. No. 659,269

'4 Claims. (Cl. 324-45 This invention relates to test circuits and more particularly to an electrical circuit capable of measuring or indicating the cathode .current cut-ofi characteristics of electron discharge devices such as cathode ray tubes.

A cathode ray tube undergoes a plurality of physical and electrical tests before it is approved by the manufacturer. When the cathode ray tube is of the type which is designated for operation as an image display device in a television receiver, one of the critical electrical tests which it must undergo occurs during determination of the value of control grid voltage necessary to cut off the beam current. It has been found that no exact value of grid bias voltage can be appropriately used to separate an .acceptable tube from those which are not acceptable, since the grid cut-01f voltage is dependent upon the electron emission characteristics of the tube.

Accordingly, an object of the invention is the evaluation of receiving tubes on the basis of cut-off characteristics which are formulated from cathode current characteristics.

A further object is the provision of a simple, eflicient and accurate test for receiving tubes.

The foregoing objects are achieved in one aspect of the invention by the provision of a test circuit utilizing a source of potential, which is proportional to the cathode current of the tube under test, for purposes of establishing a grid bias voltage. If the current cuts-oif at this bias, the tube is acceptable.

For a better understanding of the invention, reference is made to the following description, taken in conjunction with the accompanying drawings in which:

Fig. l is a schematic diagram of a cathode ray. tube and its associated testcircuit; and

Fig. 2 is a graphical reprewentation of the grid cut-01f voltage versus cathode current characteristics for a typical cathode ray tube.

Referring to the drawings, the graphical presentation indicates that the voltage required to cut-off the beam current of an acceptable cathode ray tube is proportional to the cathode current. The empirical formula which expresses this relationship may be stated as I co q where E is the grid voltage necessary to cut-off the cathode current, 1 of an acceptable tube, and p and E are constants which apply generally to cathode ray tubes. These constants may vary somewhat in actual value for different types of cathode ray tubes. It is to be understood that the values expressed in the equation are generally negative in value.

The circuit diagram shown in Fig. 1 provides means for testing a given tube for its cut-ofi characteristics in accordance with the empirical relationship illustrated in Fig. 2. The cathode ray tube 11 is shown, for purposes of illustration, as having a heater 13, cathode 15, control grid 17, screen grid 19 and an anode 21 which may be connected to the image display screen of the tube. A

ground.

, 2,947,936 I Patented 6 direct current source E, is heater :13 while direct current sources E, and B serve to supply the screen grid 19 and anode 21 respectively. A resistance 24 is coupled in series with the cathode to A switch S which may be of the double pole, double throw type, operates to connect a cathode shunting capacitance 25 to ground in one position, and to control grid 17 in the other position. When switch S is in the. position shown by the dotted lines, capacitance 25 is serially connected with voltage supply E to grid 17. It has been found that a satisfactory method of testing the control voltage beam current cut-01f characteristic of a tube utilizes a value for 1,, in the equation shown in Fig. 2. equal to the zero bias beam current of the tube. Accordingly, when switch S is in the position shown, grid 17 is at cathode potential. However, it is to be understood that a cathode current other than zero bias current may be used if desired.

In operation, switch S initially connects capacitance 25 to ground and control grid 17 directly to the cathode. The tube under test is then energized by closing S so that zero bias beam current flows through E,,, tube 11, and resistance 24. An unknown voltage, which varies proportionally with the cathode current of the tube as shown by the graph, is developed across cathode shunting capacitance 25. After this capacitance has been charged, switch S is moved so that it serially couples capacitance 25 with the known direct current supply E in a boosting relationship to grid 17. The tube is acceptable if the beam current cuts-oil when the sum of these voltages contributed by capacitance 25 and E are connected to the control grid. Stated in terms of the equation, the tube is acceptable when E is equal to or less than The screen of the tube may be observed to determine whether or not the beam current has been cut ofi.

Although the embodiment of the invention shown in the drawings is illustrated in conjunction with a cathode ray tube, it is to be understood that this type of circuit is also adaptable for use with receiving tubes other than cathode ray tubes. When testing a tube not provided with image display means, a meter could be used in the plate circuit of the tube to indicate the cathode current cut-off conditions.

Although one embodiment of the invention has been shown and described, it will be obvious to those skilled in the art that various changes and modifications may be made therein without departing from the scope of the invention as defined by the appended claims.

What is claimed is:

l. A test circuit for a grid controlled electron discharge tube comprising a voltage supply, means for developing a source of potential having a magnitude substantially proportional to the zero grid bias cathode current of the tube, and switching means for connecting said voltage supply and source of potential with the grid whereby the cathode current cut-off characteristic of the tube may be determined.

2. A test circuit for an electron discharge tube having a cathode, control grid and anode comprising a resistance coupled to the cathode, a capacitance normally shunting said resistance, means for energizing said tube to charge said capacitance in accordance with the cathode current in the tube, a voltage supply, and switching means for serially connecting said charged capacitance and voltage supply in boosting relationship with the control grid provided for energization of:

tube comprising a known voltage supply, means for developing an unknown source of potential having a magnitude substantially proportional to the cathode current of the tube, and switching means for connecting said voltage supply and source of potential to the grid whereby the ca hode c rrent cut-off characteristic of the tube may be. determine 4-. A test circuit for a grid controlled electron discharge tube which, when acceptable, has a grid cut-01f voltage equal to or less than FEQ wherein 1;, is the cathode current, B is a constant voltage supply for said grid, and T is a constant comprising means for developing -a source of potential equal to I a voltage supply equal to E and means for serially coupling said voltage supply and said source of potential in a boosting relationship to the grid whereby the cathode current cut-ofi characteristic of the tube may be determined.

References Cited in the file of this patent UNITED STATES PATENTS OTHER REFERENCES.

Radio Electronies, September 1952; pages 28-31. Tele-Tech and Electronic Industries, April 1956; pages 76-78 and 160.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No, 2,947 936 August 2 1960 Elmer 0., Stone It is hereby certified that error appears in the-printed specification of the above numbered patent requiring correction and that the said Letters Patent should read ascorrected below.

Column 3 lines 11 to 13 in the equation after F'I f' strike out the minus sign; line 15 for "1" read p line l6 for "1 read 1 -===e Signed and sealed this 11th day of April 1961:

(SEAL) Attest:

ERNEST W. SWIDER 7 ARTHUR W. CROCKER I Acting Commissioner of Patents 

